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Physiology 3120 Lecture Notes - Stroke Volume, Afterload, Parasympathetic Nervous System

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Tom Stavraky

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Human Physiology
Monday, January 18, 2010
Mechanical Performance of the Heart
Factors controlling cardiac output
Heart rate control
Stroke volume control
Preload: amount of blood in ventricle before it contracts
When EDV increases, stroke volume also increases
This occurs because highest crossbridge formation in cardiac muscle occurs when
the muscle is maximally stretched; increased force of contraction & increased
stroke volume
Frank-Starling law: increased venous retain means increased stroke volume;
adaptive function because the heart pumps out just as much as it receives
Frank-Starling curve
A (control)
B (with sympathetic stimulation); at a given EDV, stroke volume increases with
sympathetic stimulation
C (with parasympathetic stimulation); at a given EDV, stroke volume decreases
with parasympathetic stimulation
The force against which the heart has to pump is the afterload; pressure in aorta is the
afterload faced by the LV; pressure in pulmonary artery is the afterload faced by the RV
Increase in afterload causes a brief decrease in stroke volume output; after several
beats, stroke volume returns to normal
oAt the end of systole, there is a small increase in ESV
oIncreases EDV and thus stretches the ventricle; increases force of
contraction (by the Frank-Starling law), which increases stroke volume
back to normal
If the afterload becomes very high (as in severe hypertension), then the heart fails
Chronic cardiac denervation (no nerves attached to the heart; transplanted heart)
Rise in oxygen uptake and cardiac output are the same
Normal person gets rise in heart rate and little rise in stroke volume; denervated
person gets rise in stroke volume and little rise in heart rate (greater retention of
venous blood); different methods of compensation
Measurement of cardiac output
Direct Fick method
Fick principle states that the amount of a substance taken up by an organ per unit time is
equal to the (arterial level – venous level) * blood flow
Can be used to determine cardiac output (see below)
Cardiac output = oxygen consumption/[AO2] – [VO2] (rearranged formula)
Indicator dilution method
Inject known about of dye into right atrium/vena cava goes first to pulmonary
circulation, then to systemic
Take arterial blood samples
Measure the concentration of dye in the blood
Initially rises as it is taken up by the blood, then goes down as it gets taken up by
cells, and then rises for a second time (caused by recirculation)
Can extrapolate the curve back to the 0 point to obtain the duration of first
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